Polycrystalline silicon (polysilicon, poly-Si, or simply “poly”) has been a key material in the semiconductor and solar industries. It has been generally produced by a thermal decomposition and deposition process. For example, the “Siemens C-process” or “Siemens Method,” developed in the early 1950s, currently produces the majority of the polysilicon in the world. In the process, a chemical vapor deposition (CVD) reactor can be used with trichlorosilane (TCS, chemical formula SiHCl3) and hydrogen as reactant gases (working gases). At a suitable temperature, the reaction of the reactant gases occurs and produces polysilicon, which is deposited on high-purity thin silicon rods (“slim rods”) provided in the CVD reactor.
Energy consumption of the CVD process for manufacturing polysilicon is significant due to the high temperature required for the process. The silicon rods are typically heated by resistive heating by electricity. However, the efficiency of the resistive heating can be low because much of the heat generated is dissipated in the CVD reactor, with the exiting gases, and through the reactor wall. For example, the production of 1 kg of polysilicon using existing CVD reactors can require as high as 75 kWh. In addition, the deposition of silicon on the silicon rods is limited because as the silicon rods grow in dimension, the core of the rods are at a higher temperature so that the temperature at the deposition surface of the rods is sufficient for the thermal reduction of silicon. As polysilicon can melt at about 1410° C., excessive heating of silicon rods having large cross dimension can create an avalanche which causes demolition of the rods and interruption of the manufacturing process. Thus, when the diameter of the silicon rods grow to about 120 mm, the manufacturing operation is stopped and the silicon rods harvested.
To reduce radiation loss of heat in the process, it has been proposed that the internal surface of the reactors be coated with gold or silver. However, the coating of internal walls of reactor is highly technically difficult, expensive, and inconvenient as the reactor need be dismounted and delivered to the reactor manufacturer's site. Further, deposition of Si containing films on the gold or silver surface can impair the reflection properties of the gold or silver coating. Cleaning and drying operation can be time consuming, and can lower the quality of gold coating as well as result in decreased productivity of reactor.
In certain CVD reactor design, a quartz bell jar is used inside the reactor, and silicon rods are house inside the jar. Such a quartz jar typically has thin wall, through which IR radiation can easily pass. As such, additional heating can be provided to the CVD process from a heat source outside of the jar. Quartz jar having greater thickness is difficult to construct especially for commercial size CVD reactors.
Hence, there is a need to improve the existing CVD reactors to provide higher energy efficiency and productivity for the production of polysilicon.